Stars as Laboratories for Fundamental Physics - MPP Theory Group

406 Chapter 11
one needs to produce the heavy elements in the observed proportions,
and with a total amount compatible with a plausible galactic history.
It has long been held that the r-process elements were made in SN
explosions; in this case one needs a yield of about 10 −4 M ⊙ of heavy
elements per SN.
Perhaps the first realistic scenario that appears to meet these requirements
is r-process nucleosynthesis in the hot bubble between a
protoneutron star and the escaping shock wave in a core-collapse SN
explosion at a time of a few seconds after core bounce (Woosley and
Hoffmann 1992; Meyer et al. 1992; Woosley et al. 1994; Witti, Janka,
and Takahashi 1994; Takahashi, Witti, and Janka 1994; Meyer 1995).
The material in this region is very dilute because of the successful explosion,
yet very hot—around 10 9 K or 100 keV in the region where the
r-process is thought to occur. This hot bubble is not entirely empty
because of a neutrino-driven stellar wind. Therefore, one is talking
about a high-entropy environment (a few hundred k B per baryon), i.e.
a large number of photons per baryon (a few ten). For such conditions
the required neutron/proton ratio is achieved even for electron
fractions of Y e ≈ 0.40 typical for the material outside of a collapsed
SN core.
This scenario appears to be qualitatively and quantitatively almost
perfect except that the necessary combination of entropy, electron fraction
Y e , and expansion time scale do not seem to be quite born out by
current numerical calculations. Whatever the explanation of this problem,
it is fascinating that both the occurrence of a successful and sufficiently
energetic SN explosion as well as the occurrence of the r-process
in the high-entropy environment of the “hot bubble” seem to depend
crucially on the neutrino energy transfer which thus plays a dominant
role in this scenario. One may expect that r-process nucleosynthesis will
turn into a tool to calibrate the neutrino flux from a nascent neutron
star, and perhaps into a tool to study nonstandard neutrino properties
(for a first example see Sect. 11.4.5). In effect, the distribution and
quantity of r-process elements gives us a measure of SN neutrino fluxes,
independent of direct observations! This is not unlike big-bang nucleosynthesis
where the primeval light-element abundances have been an
extremely useful tool to study the properties of the primordial neutrino
heat bath (Kolb and Turner 1990). At the present time, of course, a
quantitative understanding of SN nucleosynthesis in conjunction with a
quantitative understanding of SN explosions is a field in its infancy—it
remains to be seen if it grows up to be as beautiful as big-bang nucleosynthesis.